This invention relates generally to distance measuring devices and techniques, and, more particularly, relates to a two-color synthetic Michelson interferometer for optically measuring distance over relatively long paths with resolution to a small fraction of an optical wavelength without the range ambiguity inherent in conventional interferometry.
The origins of employing an optical wavelength, presumed to be a constant, to measure absolute distance stems from the early classical experiments by Michelson and Benoit to measure the international meter in terms of the wavelength of the red line of cadmium. Laser interferometry enhances the precision with which interferometric measurement can be made, but because of its superior coherence length makes possible extended range interferometry, and through precision frequency stabilization provides the basis of an absolute wavelength standard.
The ambiguity of conventional interferometers may be greatly reduced through the use of multiple wavelengths. Measuring absolute distance interferometrically requires that the fringe order number in the interferometer be identified. One would like to employ a multiwavelength source with an ambiguity length longer than the greatest distance to be measured; however, for most practical applications this is unnecessary. Interferometer ambiguity distances large enough to be resolved by some form of a priori measurement are considered acceptable. One technique to extend the interferometer ambiguity distances employs a number of well-characterized, suitably-spaced wavelengths produced by a CO.sub.2 laser source operating in the 10.4 .mu.m wavelength band. The differences in a selected set of these wavelengths, and the differences in the differences . . . ad infinitum . . . were used to generate a hierarchy of wavelengths whereby, using fractional fringe measurement techniques, and a simple algorithm, distance employing any wavelength in the hierarchy could be established with sufficient accuracy to identify the next lower wavelength order number. By working downward through the wavelength hierarchy (from the longest wavelengths to the optical fringes), distance is ultimately established in terms of a well known optical wavelength, the unit of measure.
Analysis shows that the ideal wavelength hierarchy consists of a geometrical progression of wavelengths of sufficient density so that the fractional fringe measurement resolution of any wavelength in the hierarchy could reliably measure distance to a small fraction of the next lower wavelength. Practically, however, the availability of appropriately spaced wavelengths occurs as an act of nature. The use of isotopes can modify the available wavelengths somewhat, but this would have only a small effect on the desired progression of wavelengths.
One technique of using multiple wavelengths is disclosed in U.S. Pat. No. 4,355,899, entitled "Interferometric Distance Measurement Method" and a device for using this technique is further disclosed in U.S. Pat. No. 4,355,900, entitled "Self-Calibrating Interferometer". This particular device uses two Michelson interferometers to measure an unknown distance.
Another approach uses heterodyne photodetection to provide a basis for quantum-noise-limited operation that gives a high signal-to-noise ratios with a large signal with a small target return power. It further provides for a large variation in signal level and permits the phases (fractional fringes) of the optical signals to be measured electronically at a convenient frequency. Optical offsets necessary for heterodyne operation are provided by several Bragg cells that also serve to spectrally isolate the laser from the target returns which upset the laser's stability.
One optical setup employing heterodyning uses a CO.sub.2 laser, emitting power from both ends, where one beam is used to control and stabilize the laser and the other beam split into two beams, one of which is employed in a synthetic interferometer and the other as a local oscillator (LO) beam. Because heterodyne photodetection is used, the local oscillator beam and interferometer beams are frequency offset with Bragg cells and spatially separated. Ordinarily a single frequency translation would suffice but a single Bragg cell generates, in addition to the desired frequency-translated component, a second contaminating component due to a small backward (reflected) acoustical wave. This component has in the past been large enough to impair the phase measurement process. Employing a second Bragg cell at a second frequency provides a basis for isolating this component but increases the complexity. Two detectors are used to detect phases in the two beams which are required because of spatial separation and no frequency offsets.
One element of the prior Absolute Distance Sensor is the "two-color laser" (T-C laser), which is capable of stabilizing and operating simultaneously on any of four sets of two-color pairs (for a total of five different rotational-vibrational lines in the CO.sub.2 -10.4 .mu.m band), and of rapidly switching through the various color pairs by means of a piezoelectric mirror drive and control subsystem. All of the basic features of the T-C laser (its stability, switching capability, line pairing sequence, states-of-operation, switching speed, and derivable wavelength hierarchy) are fundamental to the operation of the Absolute Distance Sensor system.
Progressing through the optical train of the prior Absolute Distance Sensor, separate target/reference sensing beams are established at a beamsplitter and pass in the vicinity of an optical switch (chopper) employed to alternately range to the target and reference legs of the interferometer. Since it is the difference between these measurements that is of interest, phase noise in the electronics and all optics upstream from the beamsplitter is common to both measurements and may be largely cancelled. By making rapid phase measurements and by switching between target and reference at a high rate (240 Hz is used here) significant common mode noise cancellation benefits may be achieved. The target/reference beams leaving the interferometer are made colinear at the beamsplitter, and made congruent with the LO beam at a second beamsplitter, and progress to a grating where the R- and P-lines are separated and directed to separate detectors. The heterodyne photodetected signals are then digitally processed for phase. A 36 MHz clock is started and stopped by axis crossing detectors using target and reference returns from the detected R- and P-carriers as well as an electronic reference signal derived from the Bragg cell drivers, all of which are heterodyned to a convenient working frequency (in this case, 10 kHz). Statistically, the digital error of this scheme is approximately 1:2.8.times.10.sup.3 for each phase measurement. Statistical improvement is achieved by averaging over 10 cycles per phase measurement. Relative times (reference signal period and target return delay times) are directly available in a form convenient for computer input without further processing.
Although the prior Absolute Distance Sensor proved the concept of accurate distance measurement that sensor required an elaborate signal processing scheme and a complex optical system suitable only in the laboratory.